Marco Maso

Simultaneous Wireless Information and Power Transfer Under Different CSI Acquisition Schemes

In this work, we consider a multiple-input single-output system in which an access point (AP) performs a simultaneous wireless information and power transfer (SWIPT) to serve a user terminal (UT) that is not equipped with external power supply. To assess the efficacy of the SWIPT, we target a practically relevant scenario characterized by imperfect channel state information (CSI) at the transmitter, the presence of penalties associated to the CSI acquisition procedures, and non-zero power consumption for the operations performed by the UT, such as CSI estimation, uplink signaling and data decoding. We analyze three different cases for the CSI knowledge at the AP: no CSI, and imperfect CSI in case of time-division duplexing and frequency-division duplexing communications. Closed-form representations of the ergodic downlink rate and both the energy shortage and data outage probability are derived for the three cases. Additionally, analytic expressions for the ergodically optimal duration of power transfer and channel estimation/feedback phases are provided. Our numerical findings verify the correctness of our derivations, and also show the importance and benefits of CSI knowledge at the AP in SWIPT systems, albeit imperfect and acquired at the expense of the time available for the information transfer.

A Composite Approach to Self-Sustainable Transmissions: Rethinking OFDM

This paper proposes two novel strategies to extend the battery life of an orthogonal frequency-division multiplexing (OFDM) receiver by exploiting the concept of wireless power transfer (WPT). First, a new receiver architecture is devised that does not discard the cyclic prefix (CP) but instead exploits it to extract power from the received signal, realizing a WPT between the transmitter and the receiver. Subsequently, a flexible composite transmit strategy is designed, in which the OFDM transmitter transmits to the receiver two independent signals coexisting in the same band. It is shown that, by means of this approach, the transmitter can arbitrarily increase the power concentrated within the CP at the OFDM receiver, without increasing the redundancy of the transmission. Feasibility conditions for the self-sustainability of the transmission are derived, in terms of power consumption at the receiver, for both legacy and composite transmission. Numerical findings show that, under reasonable conditions, the amount of power carried in the CP can be made sufficient to decode the information symbols, making the transmission fully self-sustainable. The potential of the proposed approach is confirmed by the encouraging results obtained when the full self-sustainability constraint is relaxed, and partially self-sustainable OFDM transmissions are analyzed.

Energy-Recycling Full-Duplex Radios for Next-Generation Networks

In this work, a novel energy-recycling single-antenna full-duplex (FD) radio is designed, in which a new three-port element including a power divider and an energy harvester is added between the circulator and the receiver (RX) chain. The presence of this new element brings advantages over the state of the art in terms of both spectral efficiency and energy consumption. In particular, it provides the means of performing both an arbitrary attenuation of the incoming signal, which in turn increases the effectiveness of the state-of-the-art self-interference cancellation strategies subsequently adopted in the RX chain, and the recycling of a non-negligible portion of the energy leaked through the nonideal circulator. The performance of this architecture is analyzed in a practically relevant four-node scenario in which two nodes operate in FD and two nodes in half-duplex (HD). Analytical approximations are derived for both the achievable rates of the transmissions performed by the FD and HD radios and the energy recycled by the FD radios. The accuracy of these derivations is confirmed by numerical simulations. Quantitatively, achievable rate gains up to 40% over the state-of-the-art alternatives, in the considered scenario, are highlighted. Furthermore, up to 50% of the leaked energy at the circulator, i.e., 5% of the energy of the transmitted signal, can be recycled.

The Price of Self-Sustainability for Block Transmission Systems

In this work, the self-sustainability of block transmission systems is analyzed. In particular, orthogonal frequency division multiple access (OFDMA) is taken as a reference, due to its popularity and rather simple signal model. More precisely, a generalized variant of this scheme in which the transmitted signal is obtained as the sum of an OFDMA and a cognitive interference alignment (CIA) component, acting as an energy bearer, is considered. In this scenario, the self-sustainability of the transmission is made possible by the flexibility of the adopted strategy and the introduction of a novel energy harvesting OFDMA receiver. Both the feasibility conditions for the self-sustainability and the optimal power allocation to maximize the effectiveness of the energy transfer performed through the CIA signal are derived. Numerical results show that full self-sustainability can be achieved for several system configurations and channel statistics. However, this comes at the cost of a rate penalty with respect to a standard classic OFDMA transmission, which is termed the price of self-sustainability. A study of the relationship between the performance of both the energy and the information transfer is carried out. A CP size that minimizes the price of self-sustainability can be found for all the considered configurations.

Wireless Powered Communication Networks: Research Directions and Technological Approaches

Current wireless and cellular networks are destined to undergo a significant change in the transition to the next generation of network technology. The so called wireless powered communication network (WPCN) has been recently emerging as a promising candidate for achieving the target performance of future networks. According to this paradigm, nodes in a WPCN can be equipped with hardware capable of harvesting energy from wireless signals, that is, their battery can be ubiquitously replenished without physical connections. Recent technological advances in the field of wireless power harvesting and transfer are providing strong evidence of the feasibility of this vision, especially for low-power devices. The future deployment of WPCN is more and more concretely foreseen. The aim of this article is therefore to provide a comprehensive review of the basics and backgrounds of WPCN, current major developments, and open research issues. In particular, we first give an overview of WPCN and its structure. We then present three major advanced approaches whose adoption could increase the performance of future WPCN: backscatter communications with energy harvesting; duty-cycle based energy management; and transceiver design for self-sustainable communications. We discuss implementation perspectives and tools for WPCN. Finally, we outline open research problems for WPCN.

Optimal low-complexity self-interference cancellation for full-duplex MIMO small cells

Self-interference (SI) significantly limits the performance of full-duplex (FD) radio devices if not properly cancelled. State-of-the-art SI cancellation (SIC) techniques at the receive chain implicitly set an upper bound on the transmit power of the device. This paper starts from this observation and proposes a transmit beamforming design for FD multiple-antenna radios that: i) leverages the inherent SIC capabilities at the receiver and the channel state information; and ii) exploits the potential of multiple antennas in terms of spatial SIC. The proposed solution not only maximizes the throughput while complying with the SIC requirements of the FD device, but also enjoys a very low complexity that allows it to outperform state-of-the-art counterparts in terms of processing time and power requirements. Numerical results show that our transmit beamforming design achieves significant gains with respect to applying zero-forcing to the SI channel when the number of transmit antennas is small to moderate, which makes it particularly appealing for FD small-cell base stations.

Practical Perspectives on IoT in 5G Networks: From Theory to Industrial Challenges and Business Opportunities

The articles in this special section focus on the Internet of things applications in 5G mobile communications.

Adaptive clustering and CSI acquisition for FDD massive MIMO systems with two-level precoding

In this paper, we target a massive multiple-input/multiple-output (MIMO) system operating in frequency-division duplexing (FDD) mode, assuming the adoption of a two-level linear precoding strategy at the BS. We propose a novel strategy to effectively acquire the channel state information (CSI) at the base station (BS). In particular, we devise a cross-layer dynamic algorithm for user grouping, CSI acquisition and user scheduling that takes into account fairness considerations, application characteristics and quality of service (QoS) constraints of the users. We assess the merit of the proposed algorithm for a proportional fairness objective by comparing its performance with what is achieved by a relevant baseline algorithm in which user grouping is static and based only on the second order statistics, i.e., joint space division and multiplexing (JSDM). Our numerical findings illustrate that the proposed algorithm outperforms the baseline in terms of both fairness and speed of convergence to a steady state, and for different network topologies.

Pre-equalized faster than Nyquist transmission for 5G cellular microwave backhaul

Microwave backhaul links can nowadays deliver rather impressive throughput, however 5G systems will require further higher efficiencies and a promising solution is the so-called faster than Nyqust (FTN) transmission. A FTN communication is obtained when the Nyquist inter-symbol interference (ISI) criterion is not respected because the transmission rate is higher than that for which the shaping filter is designed. While increasing the achievable rate, the complexity increase has up to now discouraged application of FTN in practical systems. In this paper, we propose to compensate for the ISI by means of a non-linear precoding strategy, i.e. the Tomlinson-Harashima precoder (THP), in turn eliminating the need for costly equalizations at the receiver. We characterize the performance of the proposed FTN system by deriving analytical bounds on the achievable rates of the transmissions over a FTN microwave link using high-order modulations and capacity achieving codes. The proposed solution is capacity-achieving at high SNR (which is the operating regime of backhaul microwave links) and has a complexity that does not increase with the density of the constellation. Numerical results show the effectiveness of the THP-FTN solution paving the way towards its implementations on microwave links.

Cache-aided full-duplex small cells

Caching popular contents at the edge of the network can positively impact the performance and future sustainability of wireless networks in several ways, e.g., end-to-end access delay reduction and peak rate increase. In this paper, we aim at showing that non-negligible performance enhancements can be observed in terms of network interference footprint as well. To this end, we consider a full-duplex small-cell network consisting of non-cooperative cache-aided base stations, which communicate simultaneously with both downlink users and wireless backhaul nodes. We propose a novel static caching model seeking to mimic a geographical policy based on local files popularity and calculate the corresponding cache hit probability. Subsequently we study the performance of the considered network in terms of throughput gain with respect to its cache-free half-duplex counterpart. Numerical results corroborate our theoretical findings and highlight remarkable performance gains when moving from cache-free to cache-aided full-duplex small-cell networks.